Here you can find examples for join in LinQ.
C# Join[
^]
I think in your linq query the problem was caused by the difference of the type between the pd.id and od.No. The type of the two columns must be the same!
var query2 = (from pd in db.Student1s
join od in db.Test_1s on pd.id equals od.No
select new
{
od.Name,
pd.name
}).ToList();